1. Field of the Invention
The present invention relates to a method of evaluating suitable optical material for making optical elements for high-energy radiation and to the use of the optical materials obtained by this method.
2. Description of the Related Art
It is known that materials from which optical elements are made absorb more or less of the light or radiation that passes through them, so that the intensity of the light and/or the radiation is generally less after passing through an optical element than before passing through it. It is also known that the extent of the absorption depends on the wavelength of the light. The absorption in optical systems, i.e. optically transparent systems, is kept as small as possible, because these systems should have a high light permeability or transmission, at least at their respective working wavelengths. The absorption is composed of absorption from material-specific components (intrinsic absorption) and those components, which are referred to as the so-called non-intrinsic components, such as inclusions, impurities, and/or crystal defects. While the intrinsic absorption is independent of the respective quality of the material, the additional non-intrinsic components of the absorption lead to a loss of quality of the optical material.
Energy that leads to heating is absorbed by the optical material both by intrinsic and also by non-intrinsic absorption. This sort of heating of the optical material has the disadvantage that the optical properties, such as the index of refraction, change, which leads to a change in the imaging behavior in an optical component used to beam formation, since the index of refraction not only depends on the wavelength of the light but also on the temperature of the optical material. Moreover heating of an optical component leads to a change of the lens geometry. This phenomenon produces a change of the lens focal point or to blurring of the image projected with the heated lens. This leads especially in photolithography, such as is used for making computer chips and electronic circuits, to a quality impairment or to an increase in the number of rejects. That is clearly undesirable.
Furthermore it has been shown that the absorption of the material increases with time with longer irradiation with high-energy light. This effect called radiation damage is composed of a more rapidly occurring reversible component and a slower irreversible component. In the case of the more rapid radiation damage a part of the absorbed radiation is not only converted into heat, but is output again in the form of fluorescence. The formation of fluorescence in an optical material, especially in optical crystals, is also known. For example, the production and measurement of laser-induced fluorescence (LIF) in quartz, especially in OH-rich quartz, is described in W. Triebel, Bark-Zollmann, C. Muehlig, et al, “Evaluation of Fused Silica for DUV Laser Applications by Short Time Diagnostics”, Proceedings SPIE Vol. 4103, pp 1-11, 2000. Fluorescence and transmission properties of CaF2 are described in C. Muehlig, W. Triebel, Toepfer, et al, Proceedings SPIE Vol. 4932, pp. 458-466. The formation of optical absorption bands in a calcium fluoride crystal is described by M. Mizuguchi, et al, in J. Vac. Sci. Technol. A., Vol. 16, pp. 2052-3057 (1998). A time-resolved photoluminescence for diagnosis of laser damage in a calcium fluoride crystal is described by M. Mizuguchi, et al, in J. Opt. Soc. Am. B, Vol. 16, pp. 1153-1159, July 1999. The formation of photoluminescence-forming color centers by excitation with an ArF excimer laser at 193 nm is described there. However so that these sorts of measurements were possible, crystals with a relatively high impurity level were used, which are insufficient for the high requirements of photolithography. Furthermore the fluorescence measurement is performed during a time interval of 50 nsec and after the laser pulse has finished passing through the sample. It has now been shown that the fluorescence values so obtained may not be used for quality control or for determination of the extent of impurity formation and thus for formation of color centers in crystals of high quality.
Since manufacture of an entire optical component from an optical blank is very expensive and labor-intensive, there is already a need to establish the extent and nature or the radiation damage that would arise in the optical component during later usage at an earlier time point, i.e. prior to working the blank. Unsuitable material must be discarded. Attempts have already been made to determine the extent and the nature of the radiation damage of this sort by means of laser-induced fluorescence. Thus, for example, WO 2004/027395 describes a process for determination of the non-intrinsic fluorescence in an optical material. In this process the fluorescence in the optical material is directly determined with the same laser, with which the pre radiation is performed, i.e. immediately after a pre-radiation with light at an excitation wavelength of 193 nm or 157 nm.
A method for quantitative determination of the suitability of optical materials is described in DE 103 35 457 A1. In this method the energy-density-dependent transmission is measured at wavelengths in the UV by determining an equilibrium value for the transmission at different fluences, measuring the slope of the curve dT/dH for this sample and comparing with the fluorescence properties.
Laser-stable material can already be evaluated at an earlier time point during production by means of the above-described method. Photolithographic illumination devices at the present stage of development require a material, which is especially laser-stable, in their illumination optics, in the laser used in them, or their laser beam guidance system. This requirement results from the productivity requirements of this sort of apparatus, which may well increase because of increases in laser power and thus inherent increases of the energy density. The sensitivity of the aforementioned short-time measuring method for pre-evaluation of suitable optical raw material is thus no longer sufficient in order to distinguish samples with especially good laser-stability from other laser-stable samples.
Material, which should have very good properties according to its later usage, must thus be constantly tested up to now in a long duration test. When this material withstood this long duration test, it could then be further processed or worked. Typical test conditions for this sort of long duration test are, for example, irradiation with a 193 nm excimer laser with a repetition rate greater than or equal to 1000 Hz at an energy density per pulse of 15 mJ/cm2 and a total number of pulses of about 109 pulses. That means a measurement time of about 11.5 days.